This invention relates to twist drills, and more particularly but not necessarily exclusively to twist drills for producing holes of high accuracy in materials that are difficult to work, especially hard materials.
A conventional twist drill comprises a cylindrical drill body provided with a pair of helical flutes defining fluted lands between them and extending from the cutting point of the drill to a shank at the other end, by which shank the drill is securable in a chuck, for example, of a hand tool, or a drilling machine. The cutting point of the drill is of generally conical-shape, with a central chisel edge from which a pair of diametrically opposed cutting edges defined by the leading faces of the fluted lands and the flanks of the drill point that form the end faces of the fluted lands. The outer periphery of each fluted land has at its leading edge a radial projection which is variously termed a land, a cylindrical land, a wear margin, or a support margin. These two support margins extend along the length of their fluted lands and are intended to guide the drill radially as it forms a hole.
In conventional twist drills, there is a tendency to chatter, which can lead to the wall of the hole being drilled having grooves or tracks. The circularity and straightness of a hole produced by a conventional twist drill are also often inadequate for many applications. These defects are particularly pronounced when drilling very hard materials, and necessitate a subsequent reaming operation where dimensional accuracy of the hole is required.
It has been recognized that the use of only two support margins is inadequate for guiding the drill in its hole in certain applications. For example, in U.S. Pat. No. 4,913,603 there is provided a twist drill having three support margins, one at the leading edge of each of the two fluted lands and one of the fluted lands being wider than the other at its radially outer face and having a further support margin at its trailing edge to give three-point guidance. This prior art drill is also required to have cutting edges at unequal spacings. The fluted lands are therefore asymmetrical in transverse cross-section which is generally undesirable, and in any event the arrangement still does not provide adequate stability for many applications.
In British Patent 1,432,546 there is described a twist drill having symmetrical pairs of flutes and fluted lands, each of the two fluted lands having support margins at both its leading and trailing edges. Each leading edge support margin is relieved in the region nearest the drill point, so that the trailing edge support margin is required to have a cutting action widening the hole to the full diameter of the drill. This construction, however, does not overcome the instability problem encountered in drilling very hard materials.
A particular application where difficulties of accuracy of drilling have arisen is in the preparation of rock drill bits for mining applications. Rock drill bits have bodies made from very hard steels, of a generally bulbous construction, with a number of wear resistant hard metal teeth embedded in the front face of the body. The teeth or studs are inserted with an interference fit into holes drilled in that front face. At present the holes need to be drilled and then reamed to provide sufficient accuracy and a sufficiently good surface finish for the teeth or studs to be a firm fit within them. However, even the use of a reamer cannot rectify the problem caused by a hole that has not been drilled sufficiently straight.
There is therefore a need for a drill which can provide holes of improved circularity, surface finish and straightness, to close tolerances, particularly when drilling materials which are difficult to work, and especially hard materials.